The present invention relates to High Electron Mobility Transistor (HEMT) and more particularly to aluminum gallium nitride (AlGaN)/gallium nitride (GaN) HEMTs.
AlGaN/GaN HEMT (High Electron Mobility Transistor) devices are well known in the semiconductor field. U.S. Pat. Nos. 5,192,987 and 5,296,395 describe AlGaN/GaN HEMT structures and methods of manufacture. Improved HEMT structures are disclosed in commonly assigned U.S. patent application Ser. No. 09/096,967 filed Jun. 12, 1998 and entitled xe2x80x9cNITRIDE BASED TRANSISTORS ON SEMI-INSULATING SILICON CARBIDE SUBSTRATESxe2x80x9d which is incorporated by reference in its entirety.
A typical AlGaN/GaN HEMT structure 110 is illustrated in FIG. 1. A GaN channel layer 114 is formed on buffer layer 113 on a substrate 112. An AlGaN barrier layer 116 is formed on the GaN channel layer 114. A source electrode 118 and a drain electrode 120 form ohmic contacts through the surface of the AlGaN layer 116 to the electron layer that is present at the top of the GaN channel layer 114. In a conventional AlGaN/GaN HEMT, a gate electrode 122 forms a non-ohmic contact to the surface of the AlGaN layer 116.
Because of the presence of aluminum in the crystal lattice, AlGaN has a wider bandgap than GaN. Thus, the interface between the GaN channel layer 114 and the AlGaN barrier layer 116 forms a heterostructure. FIG. 2 is a band diagram showing the energy levels in the device along a portion of section I-Ixe2x80x2 of FIG. 1. As illustrated in FIG. 2, the conduction and valence bands Ec and Ev in the AlGaN barrier layer 116 are distorted due to polarization effects. Consequently, a two dimensional electron gas (2DEG) sheet charge region 115 is induced at the heterojunction between the GaN channel layer 114 and the AlGaN barrier layer 116, while the AlGaN barrier layer 116 is depleted of mobile carriers due to the shape of the conduction band. As shown in FIG. 2, the conduction band Ec dips below the Fermi level (Ef) in the area of the GaN channel layer 114 that is immediately adjacent to AlGaN barrier layer 116.
Electrons in the 2DEG sheet charge region 115 demonstrate high carrier mobility. The conductivity of this region is modulated by applying a voltage to the gate electrode 122. When a reverse voltage is applied, the conduction band in the vicinity of the sheet charge region 115 is elevated above the Fermi level, and a portion of the sheet charge region 115 is depleted of carriers, thereby preventing the flow of current from source 118 to drain 120.
As illustrated in FIG. 1, AlGaN/GaN HEMTs have typically been fabricated with coplanar metal contacts. That is, the ohmic contacts for the source 118 and drain 120 electrodes are on the same epitaxial layer (namely, the AlGaN layer 116) as the gate electrode 122. Given that ohmic contacts are intended to provide low resistance, non-rectifying contacts to a material, while the gate contact is intended to be a non-ohmic contact that blocks current at large reverse voltages, forming all three contacts on the same epitaxial layer may result in compromises between these characteristics. Stated another way, in a conventional AlGaN/GaN HEMT device, there is a tradeoff in device design when selecting the doping and composition of the AlGaN barrier layer 116 between optimizing the source and drain ohmic contacts on one hand and optimizing the non-ohmic gate contact on the other hand.
In addition, consideration should be given to providing as much current-carrying capability as possible to the sheet charge region 115 under the gate electrode 122, again, while allowing the gate to block at as high a voltage as possible. Thus, it may be advantageous to have differences in the regions between the source and gate, under the gate, and between the gate and drain in order to modify the amount of band-bending and, thus, the amount of charge. Modifying band-bending will change the amount of charge in the sheet charge region 115 as well as the electric fields present within the device.
In conventional Gallium Arsenide (GaAs) and Indium Phosphorous (InP-based) HEMT devices, an additional GaAs or Indium Gallium Arsenide (InGaAs) layer is formed on the surface of the barrier layer. Source and drain contacts are made to the additional layer, while the gate electrode is recessed down to the barrier layer. This approach, however, may not be suitable for AlGaN/GaN HEMT structures, because the top surface of GaN is generally not conductive, and there is no benefit to recessing the gate down to the barrier layer.
Thus, there is the need in the art for improvements in AlGaN/GaN HEMT structures and methods of fabricating AlGaN/GaN HEMTs.
Embodiments of the present invention provide high electron mobility transistors (HEMTs) and methods of fabricating HEMTs. Devices according to embodiments of the present invention include a gallium nitride (GaN) channel layer and an aluminum gallium nitride (AlGaN) barrier layer on the channel layer. A first ohmic contact is provided on the barrier layer to provide a source electrode and a second ohmic contact is also provided on the barrier layer and is spaced apart from the source electrode to provide a drain electrode. A cap segment is provided on the barrier layer between the source electrode and the drain electrode. The cap segment has a first sidewall adjacent and spaced apart from the source electrode. The cap segment may also have a second sidewall adjacent and spaced apart from the drain electrode. A non-ohmic contact is provided on the cap segment to provide a gate contact. The gate contact has a first sidewall which is substantially aligned with the first sidewall of the cap segment. The gate contact extends only a portion of the distance between the first sidewall and the second sidewall of the cap segment. In particular embodiments, the cap segment is a GaN cap segment.
In further embodiments of the present invention, the non-ohmic contact extends to, but not past, the first sidewall of the GaN cap segment. The GaN cap segment may have a thickness of from about 10 to about 60 xc3x85. The GaN cap segment may also be undoped GaN.
In particular embodiments of the present invention, the source electrode and the drain electrode are spaced apart a distance of from about 2 to about 4 xcexcm. Furthermore, the first sidewall of the GaN cap segment is preferably as close a possible and may, for example, be from about 0 to about 2 xcexcm from the source electrode. The second sidewall of the GaN cap segment may be from about 0.5 to about 1 xcexcm from the gate electrode.
In additional embodiments of the present invention, the AlGaN barrier layer is between about 15% and about 40% aluminum. The AlGaN barrier layer may also be doped with silicon at a concentration of up to about 4xc3x971018 cmxe2x88x923 or higher an preferably provides a total sheet concentration of up to about 5xc3x971012 cmxe2x88x922 and may have a thickness of from about 15 to about 40 nm and, preferably, about 25 nm.
In still further embodiments of the present invention, the GaN channel layer is provided on a substrate. The substrate may be silicon carbide, sapphire or the like. In particular embodiments, the substrate is 4H silicon carbide or 6H silicon carbide. Furthermore, a GaN buffer layer may be disposed between the GaN channel layer and the substrate.
In yet additional embodiments of the present invention, the gate electrode is a T-shaped gate electrode.
In method embodiments of the present invention, methods of fabricating a high electron mobility transistor (HEMT) is provided by forming a first gallium nitride (GaN) layer on a substrate, forming an aluminum gallium nitride (AlGaN) layer on the first GaN layer. A second GaN layer is patterned on the AlGaN layer to provide a GaN segment on the AlGaN layer and to expose portions of the AlGaN layer. A first ohmic contact is formed to the AlGaN layer adjacent and spaced apart from the GaN segment to provide a source electrode and a second ohmic contact is formed to the AlGaN layer adjacent and spaced apart from the GaN segment and opposite first ohmic contact such that the GaN segment is disposed between the first ohmic contact and the second ohmic contact to provide a drain electrode. A non-ohmic contact is patterned on the GaN segment to provide a gate contact. The gate contact has a first sidewall which is substantially aligned with the a first sidewall of the GaN segment adjacent the source contact. The gate contact extends only a portion of the distance between the first sidewall and a second sidewall of the GaN segment adjacent the drain contact.
In further embodiments of the present invention, the patterning of the second GaN layer and the patterning the non-ohmic contact may be provided by forming a second GaN layer on the AlGaN layer, forming a non-ohmic contact on the second GaN layer and patterning the non-ohmic contact and the second GaN layer to provide the GaN segment and the gate contact. Such patterning may further be provided by forming a mask that covers portions of the non-ohmic contact and the second GaN layer so as to define a sidewall of the non-ohmic contact and the GaN segment adjacent the source contact and a sidewall of the GaN segment adjacent the drain contact and etching the non-ohmic contact and the second GaN layer to expose portions of the AlGaN layer.